According to this story in the Guardian, a typical city of one million people poops out $13 million in (potentially recoverable) precious metals every year:
Sewage sludge contains traces of gold, silver and platinum at levels that would be seen as commercially viable by traditional prospectors. “The gold we found was at the level of a minimal mineral deposit,” said Kathleen Smith, of the US Geological Survey.
Smith and her colleagues argue that extracting metals from waste could also help limit the release of harmful metals, such as lead, into the environment in fertilisers and reduce the amount of toxic sewage that has to be buried or burnt.
“If you can get rid of some of the nuisance metals that currently limit how much of these biosolids we can use on fields and forests, and at the same time recover valuable metals and other elements, that’s a win-win,” she said.
A previous study, by Arizona State University, estimated that a city of 1 million inhabitants flushed about $13m (£8.7m) worth of precious metals down toilets and sewer drains each year.
The task of sifting sewage for microscopic quantities of gold may sound grim, but it could have a variety of unexpected benefits over traditional gold mining. The use of powerful chemicals, called leachates, used by the industry to pull metals out of rock is controversial, because these chemicals can be devastating to ecosystems when they leak into the environment. In the controlled setting of a sewage plant, the chemicals could be used liberally without the ecological risks.
